Method and apparatus for wavelength selective switch

ABSTRACT

Apparatus and method embodiments are provided for implementing a wavelength selective switch (WSS). The embodiments use combinations of switchable polarization grating (SPG) and LC cells and combinations of polymer polarization grating (PPG) and LC cells to achieve 1×N WSS systems. An embodiment optical switch includes a liquid crystal cell and a SPG cell adjacent to the liquid crystal cell. The SPG includes liquid crystal material between two photo-alignment layers, an electrode layer overlying each photo-alignment layer, and a glass substrate overlying each electrode layer. An embodiment method includes polarizing an incident light beam at a circular polarization before diffracting, at a polarization grating, the polarized incident light beam in a determined angle that corresponds to a diffraction order in accordance to the circular polarization of the incident light beam and a hologram pattern direction formed inside the polarization grating.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/592,573, filed on June Jan. 30, 2013, andentitled “Method and Apparatus for Wavelength Selective Switch,” whichapplication is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical switches, and, in particularembodiments, to an apparatus and method for a wavelength selectiveswitch.

BACKGROUND

For optical transport network equipment, the use of a reconfigurableoptical add/drop multiplexers (ROADMs) can flexibly expand the networkcapacity and reduce the operation cost. A wavelength selective switch(WSS) is one choice of technology for current RODAMs. For a 1×N WSS, 1is a common (COM) port and N represents the branch ports. The WSSoperates such that when a group of the WDM signals enter from the COMport, the signals are separated by optical wavelengths, and thenaccording to the system requirement, each wavelength is routed to one ofthe N branch ports. Conversely, the optical signals can be received asinput from N branch ports and sent as output from the COM port.

A ROADM provides an automated mechanism to flexibly add capacity asneeded without resorting to expensive and service-interrupting“forklift” upgrades. A benefit of the ROADM network is its ability toadd dynamic capacity wherever and whenever needed, with the assurancethat the underlying network automatically compensates for the addedtraffic. This eliminates the need for manual tuning or wholesaleupgrades. The ROADM can provide add/drop functions in multipledirections with multiple wavelength channels, and thus is suitable toachieve multi-directional interconnections between network rings and tobuild up mesh networks.

SUMMARY OF THE INVENTION

In accordance with an embodiment, an optical switch includes a liquidcrystal cell and a switchable polarization grating (SPG) cell adjacentto the liquid crystal cell. The SPG includes a first glass substrate, afirst electrode layer overlying the first glass substrate, aphoto-alignment layer overlying the first electrode layer, liquidcrystal material overlying the photo-alignment layer, and a secondphoto-alignment layer overlying the liquid crystal material. The firstphoto-alignment layer and the second photo-alignment layer comprisingphotosensitive polymer that have been physically altered by exposureusing two interfering light beams with opposite handedness of circularpolarization. The SPG further includes a second electrode layeroverlying the second photo-alignment layer and a second glass substrateoverlying the second electrode layer.

In accordance with another embodiment, an optical switch includes aliquid crystal cell and a polymer polarization grating (PPG) celladjacent to the liquid crystal cell. The PPG includes a glass substrate,a photo-alignment layer overlying the glass substrate and comprisingphotosensitive polymer that has been physically altered by exposureusing two interfering light beams with opposite handedness of circularpolarization, and a polymerized liquid crystal layer overlying thephoto-alignment layer on an opposite side of the glass substrate, thepolymerized liquid crystal layer has been physically altered byillumination using a uniform light beam.

In accordance with yet another embodiment, a method for operating anoptical switch comprising a polarization grating includes polarizing anincident light beam at a circular polarization, directing the polarizedlight beam to the polarization grating, and diffracting, at thepolarization grating, the polarized incident light beam in a determinedangle that corresponds to a diffraction order in accordance to thecircular polarization of the incident light beam and a hologram patterndirection formed inside the polarization grating, the hologram patterndirection formed using two interfering light beams.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIGS. 1 a and 1 b show a fabrication process of a switchablepolarization grating (SPG) cell;

FIGS. 2 a and 2 b show a SPG cell with and without applied voltage;

FIGS. 3 a to 3 c show different operation modes of a SPG cell;

FIGS. 4 a to 4 d show a fabrication process of a polymer polarizationgrating (PPG) cell;

FIGS. 5 a and 5 b show different operation modes of a PPG cell;

FIG. 6 shows an embodiment optical system for a wavelength selectiveswitch (WSS);

FIGS. 7 a to 7 h show different operation modes of a combination ofliquid crystal (LC) and SPG cells;

FIG. 8 shows an embodiment optical switch engine using combinations ofLC and SPG cells;

FIG. 9 shows another embodiment optical switch engine using combinationsof LC and SPG cells;

FIG. 10 shows yet another embodiment optical switch engine usingcombinations of LC and SPG cells;

FIGS. 11 a to 11 d show different operation modes of a combination of LCand PPG cells;

FIG. 12 shows an embodiment optical switch engine using combinations ofLC and PPG cells;

FIG. 13 shows an embodiment method for operating an optical switchengine using LC and SPG cells; and

FIG. 14 shows an embodiment method for operating an optical switchengine using LC and PPG cells.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

Currently used technologies in wavelength selective switch (WSS)products include Micro Electro Mechanical Systems (MEMS), Liquid Crystalon Silicon (LCOS), Liquid Crystal (LC) with a crystal wedge, and DigitalMicromirror Devices (DMDs). For these technologies, the optical systemscan be similar to each other with differences in the optical switchengines.

In a MEMS system, the wavelengths are diffracted to different channelsby a grating and then each wavelength is incident upon a correspondingMEMS reflection mirror. Controlling the voltage on each MEMS minor cancontrol the mirror's rotation angle to control the light reflectionangle. According to the network's requirements, each wavelength can bereflected to a defined angle. The reflected wavelength beams with sameangles from multiple channels can be diffracted into one beam afterpassing through the grating a second time and then coupled into anoutput port. In order to control the light attenuation and realizehitless function during switching, each MEMS mirror has two rotationaldirections, one rotation for port switching and another rotation forattenuation and hitless control. A MEMS based WSS has advantages ofsimple optical system and good performance. However, it has severaldisadvantages such as high cost on MEMS chip manufacturing due torelatively low yield, high cost on electronics due to the requirement ofhigh voltage driving for MEMS mirrors, difficulty to realize largenumbers of ports, and design difficulty to a flexible grid (Flexgrid)function.

LCOS is another technology that is used in WSS systems. The LCOS iscomposed of a LC layer that is positioned between a glass substrate anda silicon backplane. In a LCOS based WSS, each wavelength light,separated by a grating, is incident on the LCOS panel and covers M×Npixels. Through controlling the voltages on these pixels, a LC phasegrating can be formed so that the incident light beam is diffracted to adefined angle. Changing the LC grating pitch can result in differentdiffraction angles. Therefore, controlling LC phase grating pitch for awavelength light can route the light beam to the defined output port. ALCOS based WSS has several advantages such as simple optical system,easy to realize high port account, and easy to realize a Flexgridfunction. The disadvantages include complicated electronic drivingscheme, substantially complex control software, difficulty to realizelow cross-talk, and relatively high temperature sensitivity.

Another technology used in WSS is LC with a crystal wedge. WSS designusing LC with crystal wedge is described in U.S. Pat. No. 7,499,608issued Mar. 3, 2009, and entitled “Apparatus And Method for OpticalSwitching with Liquid Crystals And Birefringent Wedges”. The switchengine of such a WSS consists of several stages of LC cell and wedgeplate combination, depending on the required number of output ports. Ineach stage, the LC cell is used to switch light polarization and thewedge plate is used to refract the light to two directions depending onthe polarization of the incident light, resulting in a 1×2 opticalswitch. Therefore, a stack of N stages results in a 1×2^(N) opticalswitch. The LC cell used is separated to M pixels that are defined bythe required optical channels. Controlling the voltage on LC pixels canroute the corresponding wavelength light to the defined output ports.Such WSS has advantages of simple driving electronics, high vibrationresistance, and high reliability. The disadvantages include high costdue to high material cost, relatively low yield due the complicateddevice assembly process, and difficulty to realize high port count.

DMD technology is also used in WSS systems. In such system, eachwavelength light is incident upon several DMD MEMS minors. Controllingrotation angles of these minors can direct a light beam to the definedangles. Since the minors only have two deflection positions, one DMDchip based WSS only can realize a 1×2 switch. To increase the switchingports of a WSS, more DMD chips are needed, resulting in high cost andhigh difficulty in optical system design.

As described above, the WSS systems using existing technologies havedisadvantages including complex driving electronics with complexsoftware, high cost of materials, low resistance to vibration, anddifficulty to expand to a large number of ports. Described herein areembodiment systems and methods for implementing a WSS. The differentembodiments use combinations of switchable polarization grating (SPG)and LC cells and combinations of polymer polarization grating (PPG) andLC cells to achieve 1×N WSS systems overcoming at least some of thedisadvantages of the systems above.

FIGS. 1 a and 1 b show a fabrication process 100 of a SPG cell. In aconventional LC cell fabrication, the LC alignment layer is fabricatedby rubbing or photo-exposing two polymer layers coated on twosubstrates, which are used to sandwich the LC. The fabrication process100 of a SPG cell is different with respect to forming the LC alignmentlayer. In a first step (FIG. 1 a) of the fabrication process 100 of theSPG cell, two photosensitive polymer layers 102 are coated on two glasssubstrates 106, respectively, and then two glass substrates are puttogether, leaving a gap for LC filling. An electrode (conductor) layer104 is also added between each photosensitive polymer layer 102 andrespective glass substrate 106. Next (FIG. 1 b), two interferenceultra-violet (UV) light beams 192 (at suitable incident angles) withopposite handedness of circular polarization (with right-handed andleft-handed circular polarization respectively) is used to expose (e.g.,through the glass substrates 106) the two polymer layers 102 to form aholographic pattern in the polymer layers 102. This interference beamexposure may be applied on each side of the SPG cell to form analignment layer from the photosensitive polymer layer 102. When LC 108is filled into the gap and sandwiched between the two glass substrates,the molecules of the LC 108 are aligned with the hologram pattern formedon the photosensitive polymer layers 102 that now serve as LC alignmentlayers.

FIGS. 2 a and 2 b show a SPG cell 200 with and without applied voltage.The SPG cell 200 may be fabricated using the fabrication process 100.Without an applied voltage to the electrode layers 204 (FIG. 2 a), theLC 208 in the SPG cell 200 forms a grating that causes incident light onany of the glass substrates 206 to be diffracted to a directiondetermined by the angle of the two exposing beams (during thefabrication process 100) to form the alignment layers 202. When anon-zero voltage is applied to the electrode layers 204 (FIG. 2 b), theLC 208 molecules become aligned with the electrical field caused by theapplied voltage, and hence the LC grating effect (caused by thealignment layers 202) is cancelled out and incident light on any of theglass substrates 206 is no longer diffracted. To cancel the LC gratingeffect, a sufficiently high voltage may be needed, for example above athreshold voltage (V_(th)).

The SPG cell above has three diffraction orders of 0 and ±1 that aredifferent from general gratings. FIGS. 3 a to 3 c show differentoperation modes 300 of the SPG cell. Each operation mode corresponds toa diffraction order, and each order is diffracted into a differentangle. With a sufficiently high voltage applied to the SPG cell (FIG. 3a), the light is diffracted into the 0^(th) order no matter what theincident light polarization is. When no or low voltage is applied (FIGS.3 b and 3 c), the diffracted light direction is dependent on theincident light polarization. An incident light beam with right-handedcircular polarization is diffracted to the +1^(st) order (FIG. 3 a),while incident light beam with left-handed circular polarization isdiffracted to the −1 ^(st) order (FIG. 3 c). After being diffracted bythe SPG cell, the light's handedness of polarization is changed(switched between right-handed and left-handed circular polarizations),as shown in FIGS. 3 b and 3 c.

FIGS. 4 a to 4 d show a fabrication process 400 of a PPG cell. A firststep (FIG. 4 a) of the fabrication process 400 of the PPG cell is tocoat a photo-alignment layer 402 on a glass substrate 406. A second step(FIG. 4 b) is to expose the polymer layer 402 with two interference UVbeams (492) with opposite handedness of circular polarization. A thirdstep (FIG. 4 c) is to coat a polymerizable LC layer 403 on the top ofthe photo-alignment layer 402. A forth step (FIG. 4 d) is to use auniform UV beam 494 to illuminate the polymerizable LC layer 403 topolymerize the LC composition (molecules) of the layer. Thus, a polymergranting is formed on the glass substrate 406.

The resulting PPG cell is a fixed grating in that its diffractioncharacteristics cannot be changed through applying voltages (as in thecase of the SPG cell above). FIGS. 5 a and 5 b show different operationmodes 500 of the PPG cell. Each operation mode corresponds to adiffraction order, and each order is diffracted into a different angle.An incident light beam is diffracted into one of two directions.Specifically, incident light beam with right-handed circularpolarization is diffracted to the +1^(st) order (FIG. 5 a), whileincident light beam with left-handed circular polarization is diffractedto the −1^(st) order (FIG. 5 b). In either case after diffraction, thepolarization handedness of the beam is changed or switched to theopposite handedness.

FIG. 6 shows an embodiment optical system 600 for a WSS. The WSS opticalsystem 600 includes a fiber array 601, a micro lens array 602, a beamdisplacer array 603, a half wave plate array 604), a cylindrical lens605, a cylindrical reflection mirror 606, a grating 607, and an opticalswitch engine 608. The components of the WSS optical system 600 can bearranged as shown in FIG. 6 or in any other suitable arrangement thatachieves the same or similar functionality. In other embodiments,additional components that may be similar or different than thecomponents above may also be used. Some of the components above may alsobe replaced by combinations of same or other components that achieve thesame functionality.

The fiber array 601 is used for input port and output ports. When aninput or incident light beam from one fiber 601 passes through the microlens array 602, the beam displacer array 6033, and the half wave platearray 604, the beam is separated into two parallel beams with identicallinear polarization state. The two light beams then become collimatedbeams after passing through the cylindrical lens 605 and the cylindricalreflection mirror 606. The light beams are then diffracted by thegrating 607, resulting in separated wavelengths. Each wavelength is thenfocused on the optical switch engine 608. The switch engine 608 routeseach wavelength to a defined port. The corresponding optical beams passthrough the optical system 600 again (in a reverse order of components)and are coupled into defined output fibers.

The optical switch engine 608 of the WSS optical system 600 can beimplemented using a suitable WSS system that includes combinations ofSPG and LC cells or PPG and LC cells, as described below. In comparisonto other used WSS technologies (e.g., MEMS, LCOS, LC and wedge plate,DMD), the WSS system using SPGs or PPGs has advantages of simple opticalsystem, simple electronic driving circuit, high reliability, highperformance, easily achieved high port count, and low product cost.

FIGS. 7 a to 7 h show different operation modes 700 of a combination ofLC and SPG cells. A LC cell 710 is positioned before a SPG cell 720(with respect to incident light). The LC cell 710 is used to control orswitch the light polarization and the SPG cell 720 is used to diffractthe light beam to a defined direction.

As shown in FIGS. 7 a, 7 c, 7 e, and 7 h, when a relatively high voltage(VH) (e.g., above a threshold) is applied on the LC cell 710, theincident light beam polarization is not changed through the LC cell. Asshown in FIGS. 7 b, 7 d, 7 f, and 7 g, without applied voltage or with arelatively low voltage (VL) (e.g., below a threshold) on the LC cell710, the incident light beam polarization is switched betweenright-handed and left-handed polarization. As shown in FIGS. 7 a, 7 b, 7e, and 7 f, when a relatively VH (e.g., above a threshold) is applied onthe SPG cell 720, the light beam is diffracted to the 0^(th) order, nomatter what is the polarization of the input light. As shown in FIGS. 7c, 7 d, 7 g, and h, without applied voltage or with a relatively VL(e.g., below a threshold) on the SPG cell 720, the light beam can bediffracted to either the +1^(st) order or the −1^(st) order, dependingon the incident light's polarization that is controlled by the LC cell710. Regardless whether the input light has right-handed or left-handedcircular polarization, the combination of the LC cell 710 and the SPGcell 720 can route the light beam to three directions, resulting in a1×3 optical switch. N groups of LC and SPG cells can realize a 1×3^(N)optical switch.

FIG. 8 shows a cross section of an embodiment optical switch engine 800using combinations of LC and SPG cells. The optical switch engine 800can be used as the optical switch engine 608 in the WSS optical system600. The optical switch engine 800 comprises a variable opticalattenuator (VOA) 805 including a LC cell 810 coupled to a polarizer 815,a quarter wave plate (QWP) 840, a 1×9 optical switch 830 including twoconsecutive pairs of LC 810 and SPG 820 cells, and a prism or mirror890. The components can be arranged as shown in FIG. 8 or in anothersuitable order. The LC cells 810 and SPG cells 820 can have M pixels inthe perpendicular direction to the N=9 beams (perpendicular to thesurface of FIG. 8). In FIG. 8, N is the number of beams corresponding toports and M is the number of pixels corresponding to wavelengthchannels. LC cells used in the optical engine 800 can be electricallycontrolled birefringence (ECB), twisted nematic (TN), and verticallyaligned (VA) cells.

For simplicity, the switch engine's working principle is described forone wavelength, as shown by the cross section of the engine 800 in FIG.8. However, the same working principle applies to all M pixels.

The input light first passes through the VOA 805 that is used to controlthe light power attenuation. Controlling the voltage on the LC cell 810can control the output optical power of the VOA 805. The QWP 840 is usedto change the linear polarization of the light into a circularpolarization. The light beam then passes through two groups of LC 810and SPG 820 cells (the 1×9 optical switch 830). Thus, the output beamhas 9 possible angles with the optical axis. The beam is then reflectedby the prism or mirror 890 and becomes parallel to the optical axisafter passing through the switch 830. The optical switch engine 800 canbe designed properly to achieve about equal distance between any twoadjacent light paths (of the 9 possible switching angles). As such, astandard fiber array can be used as the optical output ports (e.g., with9 output ports).

FIG. 9 shows a cross section of another embodiment optical switch engine900 using combinations of LC and SPG cells. The optical switch engine900 can be used as the optical switch engine 608 in the WSS opticalsystem 600. The optical switch engine 900 comprises a VOA 905 includinga LC cell 910 coupled and a polarizer 915, a 1×7 optical switch 930including a pair of LC 910 and SPG 920 cells followed by a second SPGcell 920, and a prism or mirror 890. The components can be arranged asshown in FIG. 9 or in another suitable order. The LC cells 910 and SPGcells 920 may also have M pixels in the perpendicular direction to theN=7 beams (perpendicular to the surface of FIG. 9). One differencebetween the optical switch engine 900 and the optical switch engine 800is that the optical switch engine 900 uses one LC cell 910 and two SPGcells 920 to achieve a 1×7 optical switch. In the 1×7 optical switch,the LC cell 910 is used to control the light polarization and the twoSPG cell 920 are used to diffract light to the defined angles.Additionally, the optical switch engine 900 does not include a QWP.Instead, the LC cell 910 in the VOA 905 is designed as a switchablequarter wave plate (switching between λ/4 and 3 λ/4) to change thelinear polarization of the incident light into a circular polarization.To increase optical output ports, more SPG cells 920 can be added to theoptical switch engine 900, e.g., in front of the minor or prism 990. Forexample, with N SPG cells 920, a 1×(2^(N+1)−1) optical switch engine canbe implemented.

FIG. 10 shows a cross section of yet another embodiment optical switchengine 1000 using combinations of LC and SPG cells. The optical switchengine 1000 can be used as the optical switch engine 608 in the WSSoptical system 600. The optical switch engine 1000 comprises a VOA 1005including a LC cell 1010 coupled to a polarizer 1015, a QWP 1040, a 1×8optical switch 1030 including three SPG cells 1020, and a prism ormirror 1090. The components can be arranged as shown in FIG. 10 or inanother suitable order. The SPG cells 1020 may also have M pixels in theperpendicular direction to the N=8 beams (perpendicular to the surfaceof FIG. 10). Unlike the optical switch engines 800 and 900 above, theoptical switch engine 1000 only uses SPG cells 1020 to control the lightdiffraction angles without a LC cell. After a light beam passes throughthe VOA 1005 and the QWP 1040, the linear polarization of the incomingbeam is changed to the circular polarization. Each SPG cell 1020 candiffract the light beam to two possible angles. Therefore, with N SPGcells, a 1×2^(N) optical switch engine can be formed.

FIGS. 11 a to 11 d show different operation modes 1100 of a combinationof LC and PPG cells. A LC cell 1110 is positioned before a SPG cell 1150(with respect to incident light). The LC cell 1110 is used to control orswitch the light polarization and the PPG cell 1150 is used to diffractthe light beam to one of two possible directions depending on theincoming light's polarization. Therefore, N groups of LC and PPG cellscan compose a 1×2^(N) optical switch.

As shown in FIGS. 11 a and 11 d, when a VH (e.g., above a threshold) isapplied on the LC cell 1110, the incident light beam polarization is notchanged through the LC cell. As shown in FIGS. 11 b and 11 c, withoutapplied voltage or with a VL (e.g., below a threshold) on the LC cell1110, the incident light beam polarization is switched betweenright-handed and left-handed polarization. As shown in FIGS. 11 a and 11c, when the incident light on the PPG cell 1150 has a right-handedcircular polarization, the light beam is diffracted to the +1^(st)order. As shown in FIGS. 11 b and 11 d, when the incident light on thePPG cell 1150 has a left-handed circular polarization, the light beam isdiffracted to the −1^(st) order.

FIG. 12 shows a cross section of an embodiment optical switch engine1200 using combinations of LC and PPG cells. The optical switch engine1200 can be used as the optical switch engine 608 in the WSS opticalsystem 600. The optical switch engine 1200 comprises a VOA 1205including a LC cell 1210 coupled to a polarizer 1215, a 1×4 opticalswitch 1230 including two consecutive pairs of LC 1210 and PPG 1250cells, and a prism or minor 1290. The components can be arranged asshown in FIG. 12 or in another suitable order. The LC cells 1210 and PPGcells 1250 can have M pixels in the perpendicular direction to the N=4beams (perpendicular to the surface of FIG. 12). Similar opticalswitches can be designed to have any number of output ports by stackingtogether a required number of LC and PPG pairs.

FIG. 13 shows an embodiment method 1300 for operating an optical switchengine using LC and SPG cells. For example, the method 1300 isimplemented using any of the optical switch engines 800, 900, and 1000.At step 1310, an incident light beam is polarized in a left-handed orright-handed circular polarization. For example, the linearly polarizedincident light beam is converted into a circularly polarized light usingthe QWP 840 or 1040 or the electrically switchable (by applied voltage)LC. At step 1320, the circularly polarized light beam is diffractedusing at least one SPG cell. The diffracted light beam's handedness isalso switched. For example, the circularly polarized light is switchedbetween left-handed and right-handed direction using a firstelectrically switchable LC 810 in the 1×9 optical switch 830 (or LC 910in the 1×7 optical switch 930) and subsequently diffracted in acorresponding angle by a next electrically switchable SPG 820 (or 920).In another example, the circularly polarized light is directlydiffracted in a corresponding angle by a first electrically switchableSPG 1020 in the 1×8 optical switch 1030.

FIG. 14 shows an embodiment method 1400 for operating an optical switchengine using LC and PPG cells. For example, the method 1400 isimplemented using the optical switch engine 1200. At step 1410, anincident light beam is polarized in a left-handed or right-handedcircular polarization. For example, the linearly polarized incidentlight beam is converted into a circularly polarized light using theelectrically switchable LC. At step 1420, the circularly polarized lightbeam is diffracted using at least one pair of LC and PPG cells. Thediffracted light beam's handedness is also switched. For example, thecircularly polarized light is switched between left-handed andright-handed direction using a first electrically switchable LC 1210 inthe 1×4 optical switch 1230 and subsequently diffracted in acorresponding angle by a next electrically switchable SPG 1250.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. An optical switch comprising: a liquid crystal cell; and a switchable polarization grating (SPG) cell adjacent to the liquid crystal cell, the SPG comprising: a first glass substrate; a first electrode layer overlying the first glass substrate; a photo-alignment layer overlying the first electrode layer; liquid crystal material overlying the photo-alignment layer; a second photo-alignment layer overlying the liquid crystal material, the first photo-alignment layer and the second photo-alignment layer comprising photosensitive polymer that have been physically altered by exposure using two interfering light beams with opposite handedness of circular polarization; a second electrode layer overlying the second photo-alignment layer; and a second glass substrate overlying the second electrode layer.
 2. The optical switch of claim 1, further comprising: a variable optical attenuator (VOA) comprising a second liquid crystal cell and a polarizer; a quarter wave plate positioned between the VOA and the liquid crystal cell; a prism or minor positioned next to the SPG cell on an opposite side from the liquid crystal cell; and one or more pairs of an additional liquid crystal cell and an additional corresponding SPG cell positioned between the SPG cell and the prism or minor, wherein the optical switch is a 1×3^(N) optical switch configured to optically connect one common port to 3^(N) separate ports, where N is a number of pairs of liquid crystal cells and corresponding SPG cells in the optical switch.
 3. The optical switch of claim 1, further comprising: a variable optical attenuator (VOA), comprising a second liquid crystal cell and a polarizer; a prism or minor positioned next to the SPG cell on an opposite side from the liquid crystal cell; and one or more additional SPG cells positioned between the SPG cell and the prism or minor, wherein the optical switch is a 1×(2^(N+1)−1) optical switch configured to optically connect one common port to 2^(N+1)−1 separate ports, where N is a number of SPG cells in the optical switch.
 4. The optical switch of claim 3, wherein the liquid crystal cell is configured as a switchable quarter wave plate to convert an incident light polarization from linear polarization into right-handed or left-handed circular polarization.
 5. The optical switch of claim 1, further comprising: a variable optical attenuator (VOA), comprising the liquid crystal cell and a polarizer; a quarter wave plate positioned between the VOA and the SPG cell; a prism or minor positioned next to the SPG cell on an opposite side from the VOA; and one or more additional SPG cells positioned between the SPG cell and the prism or minor, wherein the optical switch is a 1×2^(N) optical switch configured to optically connect one common port to 2^(N) separate ports, where N is a number of SPG cells in the optical switch.
 6. The optical switch of claim 1, wherein both the liquid crystal cell and the SPG cell comprise a plurality of pixels corresponding to wavelength channels and aligned perpendicular to a direction of a plurality of parallel light beam paths through the optical switch corresponding to optical switch ports, and wherein the optical switch is designed to have equal distance between parallel output light beams from the optical switch.
 7. The optical switch of claim 1, further comprising: a fiber array that transmits and receives one or more incident light beams to and from the liquid crystal cell and the SPG cell; a micro lens array positioned on the optical path next to the fiber array; a beam displacer positioned on the optical path next to the micro lens array; a half wave plate array positioned on the optical path next to the beam displacer; a cylindrical lens positioned on the optical path between the half wave plate array and the liquid crystal cell with the SPG cell; a cylindrical reflection minor facing the liquid crystal cell and the SPG cell behind the cylindrical lens on the optical path and positioned to reflect a light beam that is passed through the cylindrical lens back through the cylindrical lens; and a grating facing the cylindrical reflection minor behind the cylindrical lens on the optical path and positioned to diffract a light beam that is passed through the cylindrical lens back onto the cylindrical lens.
 8. The optical switch of claim 1, wherein the SPG cell is configured to, with no applied voltage or a first applied voltage between the first electrode layer and the second first electrode layer, diffract an incident light beam that has a circular polarization in a determined direction and reverse the circular polarization's handedness of the diffracted incident light beam.
 9. The optical switch of claim 8, wherein the SPG cell is configured to, with a second applied voltage between the first electrode layer and the second first electrode layer, pass an incident light beam through the SPG cell without diffraction and without change in polarization.
 10. The optical switch of claim 1, wherein the liquid crystal cell is configured to, with no applied voltage or a first applied voltage across the liquid crystal cell, pass an incident light beam that has a circular polarization to the SPG cell after reversing the circular polarization's handedness and alternatively, with a second applied voltage across the liquid crystal cell, pass the incident light beam without changing the circular polarization's handedness. 